EE529 Semiconductor Optoelectronics Semiconductor Lasers 1. Optical - PowerPoint PPT Presentation

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers EE529 Semiconductor Optoelectronics Semiconductor Lasers 1. Optical gain spectrum 2. Laser threshold, power and efficiency 3. Modulation characteristics 4. Advanced laser structures Reading: Liu, Sec. 13.3-13.4, 13.9-13.10 Ref: Bhattacharya, Sec. 6.7, 7.2, 7.13; Liu, Sec. 4.3, 5.1 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Laser Operation Lasing process in a ruby laser Process: Population inversion ― Through optical pumping or carrier injection. 1. Seed photons ― From spontaneous emission and initiate the stimulated emission process. 2. Optical cavity ― Resonant enhancement and define the output wavelength. 3. 4. Gain saturation – Population inversion decreases as stimulated emission increases. → Steady state. Output coupling ― Let some of the photons out at each round trip. 5. 2 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Basic Semiconductor Laser Structure n + p + Junction E c n + p + E g eV o E c E Fn Inversion E c region E v E g Holes in V B E Fn E Fp eV Electrons in C B Electrons E c E Fp E v (a) (b) Current V Cleaved surface m irror Photograph of an edge emitting laser diode L Electrode G aA s p + L n + G aA s Electrode Active region 500 µ m (stim ulated em issio n region) 3 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Different Regimes of Operation E n e r g y O p tic a l g ain E Fn − E Fp C B Pumped electrically or E Fn E le c tron s optically until population E c in C B inversion happened. e V h υ 0 → Emission > Absorption. E g H o le s in V B E v = E m pty s ta tes A t T > 0 E Fp V B A t T = 0 O p tic a l ab so rp tio n D e n sity o f sta te s L ase r Optical Power O ptic a l P ow e r L E D Optical Power S tim ulate d λ e m issio n L ase r Optical Power S ponta ne ous λ e m issio n I 0 I th λ 4 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Optical Gain Spectrum Optical gain coefficient g: Fractional change in the light power (or intensity) per unit distance * 3/2 2 ( ) 2( ) m c 1/2 ν = ν − − g( ) r [ ( ) ( )] h E f E f E ν τ 2 1 g c v 2 2 2 n h sp ∆ E F 5 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Threshold Gain g th Shown on the right is a typical output power-Injection current P O characteristics of a laser diode. When does lasing happen? Steady-state condition: Laser power is constant → Round -trip gain = Round-trip loss Lasing R e f le c tin g P f P i R e f le c tin g I I th → g th su rf a c e su rf a c e E f E i 2 S te a d y sta te E M o sc illa tio n s C a v ity a x is x 1 R 2 R 1 L Where does the loss come from? Reflection loss at the mirrors. Losses in the cavity medium (scattering at defects, absorption by impurities, absorption by free carriers …) = −α 2 exp[ (2 )]exp[ (2 )] P PR R g L L f i 1 α : Loss coefficient of the laser medium   1 1 = → = α +   ln P P g f i th 2   L R R 1 2 6 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Threshold Current and Efficiency Laser diode active layer thickness = d Determine N th from the gain spectrum ( ) edR N edN = = η τ sp th th J J or η η th th inj s inj i Above threshold, under steady state, ν (1/ 2 )ln(1/ ) h L R R = η η − 1 2 ( ) P I I α + 0 ut i inj th (1/ 2 )ln(1/ ) e L R R 1 2 External quantum efficiency ν ( / ) (1/ 2 )ln(1/ ) d P h L R R η = = η η out 1 2 ( ) − α + e i inj / (1/ 2 )ln(1/ ) d I I e L R R 1 2 th Slope efficiency n P o ν dP h η = = η out Threshold population n th s e VdI eV n inversion Power conversion efficiency ∝ N ph P o = Lasing output pow er ν   P h I η = = η − Slope represents efficiency out th 1   I c e   VI eV I I th 7 Lih Y. Lin

EE 529 Semiconductor Optoelectronics Exercise: Threshold Current Density of a – Semiconductor Lasers GaAs Laser Calculate the threshold current of a GaAs laser with an undoped active region of width d = 0.2 µm starting from g(E) and R sp (E) : ( ) 1/2 − E E ( ) 1/2 − − − − = × − − = × g − 29 1 3 1 4 1 ( ) 1.15 10 ( )[1 ( )] s cm (eV) g( ) 3 10 [ ( ) ( )] cm R E E E E f E f E E f E f E 2 1 2 1 sp g c v c v E Assuming the following parameters for the laser: L = 400 µm, R 1 = R 2 = 0.9, α = 10 3 cm -1 , Γ = 0.95, η i = 0.9, η inj =1 . Along your calculation, verify that you obtain the following results: 8 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Exercise: Efficiencies of an InP Laser λ = An InP Fabry-Perot laser emits at wavelength ~ 920 nm. It has an η = injection efficiency and an internal quantum efficiency 90% inj η = 95% . The voltage applied to the device is 2.5V. The reflectivity i R = R = of both cavity ends are 100% and 70% . The loss 1 2 α = cm -1 . Its threshold current is coefficient of the laser medium is 1 = mA. characterized to be 1 I th (a) The semiconductor laser has a cavity length L = 400 µm. Assume the refractive index n = 3.3. What should be the exact emission wavelength? At which longitudinal mode does lasing occur? (b) Calculate the photon extraction efficiency, external quantum efficiency, and slope efficiency of the device. (c) If the laser is operated at an injection level twice the threshold, find its power conversion efficiency and output power. 9 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Transient Response Transient phenomena occur because of the time required for the electron and photon populations to come into equilibrium. As the photon population builds up rapidly, the carrier density is depleted until it falls below transparency condition.  Photon population decreases.  Carrier population starts to build up again, this time from a higher initial value, and so does the photon population. 10 Lih Y. Lin

EE 529 Semiconductor Optoelectronics Frequency Response under – Semiconductor Lasers Small-signal Modulation As the initial oscillations cease, the situation becomes periodic small-signal modulation of a laser. γ γ m Ω = ϕ = Ω − Ω + Ωγ i Complex response function: c n ( ) | | r r e 2 2 i r r γ γ 2 2 2 m = 2 = Modulation power spectrum: ( ) ( ) c n R f r f π − + π γ 4 2 2 2 2 2 2 16 ( ) 4 f f f r r 1/2  γ  2 = − 2  r  Resonance peak: f f 3-dB modulation bandwidth: π pk r 2  8  1/2   γ 2 = + − 1/2 2  r  (1 2) f f 3 dB r π  2  8 2 11 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Exercise: Modulation Characteristics A GaAs QW VCSEL has the following parameters: Emission wavelength = 850 nm, n = Γ = 3.52 0.2 refractive index , gain overlap factor , threshold gain coefficient = × − τ = 4 1 8.16 10 m , excess carrier spontaneous lifetime , gain cross-section g 3.02 ns th s − σ = × 19 2 2.2 10 m . γ and cavity decay rate γ . (a) Find the values of spontaneous carrier relaxation rate s c τ ? What is the photon lifetime c = × − = µ 18 3 (b) If the laser output power 60.6 W , the mode volume 4.74 10 m , P V mode out η = the emitted and photon extraction efficiency . Find the intracavity photon 89% t density S . 0 (c) Find the values of differential gain parameter g . Assume the nonlinear gain n = − γ and parameter , find the values of differential carrier relaxation rate g g p n n γ . nonlinear carrier relaxation rate p (d) Find the values of relaxation resonance frequency f and total carrier relaxation rate r γ . r (e) Find the resonance peak of the modulation spectrum . What is the 3-dB f pk modulation bandwidth ? f 3 dB 12 Lih Y. Lin

EE 529 Semiconductor Optoelectronics – Semiconductor Lasers Heterojunction Lasers p n p I th for homostructure laser diode extremely high → Not practical for room temperature operation G a A s A l G a A s A l G a A s (a) µ m ) (~ 0 .1 Heterostructure laser: Enhance carrier E le c tro n s i n C B E c ∆ E c confinement and photon confinement E c 2 eV 1.4 eV 2 eV Cleaved reflecting surface (b) E v W E v H o l e s in V B L R e fra c t iv e Stripe electrode in d e x ∆ n ~ 5 % Oxide insulator A c t iv e (c) p -GaAs (Contacting layer) re g io n p -Al x Ga 1-x As (Confining layer) P h o to n d e n si ty p -GaAs (Active layer) n -Al x Ga 1-x As (Confining layer) 2 1 3 Substrate n -GaAs (Substrate) Current Substrate paths (d) Electrode Cleaved reflecting surface Elliptical laser Active region where J > J th . beam (Emission region) 13 Lih Y. Lin